Planar antennas have found a widespread use not the least in the area of mobile communication. A classical type is the basic patch antenna being a square conduction surface with a side length of λ/2, wherein λ is wavelength, see FIG. 1. The conducting surface is provided spaced apart from a ground plane in the form of a very large conducting surface and with air between the conducting surface and the ground plane.
Many derivatives from the basic patch antenna are in use. In one class the basic λ/2-size is retained but bandwidth has been improved by slots of various shapes, manufacturing has been facilitated, the antenna has been adopted for multi-band use or for different polarizations etc.
Another class of prior art derivatives has a size much smaller than λ/2 and the improvements in this class have been concentrated to improvements of the usually poor bandwidth and to multi-band performance. The generic name “Small Patch Antennas” or simply SPAs is used herein as a common name for this kind of antennas which is justified by the fact that all small patch antennas have a number of important problems and solutions in common.
The small patch antennas are like most patch antennas resonant structures wherein different means have been used to tune down the frequency from what could be expected from their size. When the main current is restricted to one direction the square patch can be made narrower, as shown in FIG. 2, and its length can also be reduced by 50% by a grounding in the former middle (or ground potential) and cutting one half as shown in FIG. 3. These changes will make the surface considerable smaller than the original λ/2 by λ/2, such as a tongue of λ/4 times λ/10, see FIG. 3, but this 90% area reduction is achieved at the expense of bandwidth performance. Even a length of λ/4, corresponding to a length of around 80 mm at the common telephone frequency bands in the 800 to 1000 MHz range, is many times too large for a mobile phone when considering the customers demands for small and light weight telephones.
One common antenna element in the class “small patch antennas” is the PIFA element meaning Planar Inverted F-Antenna, where the F-antenna is a common short-wave antenna type used among radio amateurs. A majority of built in telephone antennas today are said to be of this type or some “modified PIFA” type. The “basic PIFA” is a λ/4 long strip connected to a ground-plane below the strip at one end and open at the other end. An input connection is located at a place in between the open end and the grounded end to get desired input impedance, which typically is chosen to 50 Ohms. One property common for all resonant structures is that there is a free choice of input impedance by a suitable feeding point. For applications where λ/4 is too long the length of the PIFA can be made shorter. FIG. 4 shows a basic PIFA configuration. Pure downscaling in size would increase the resonance frequency correspondingly but the resonance frequency can be tuned down in many ways to get the desired resonance frequency. Three typical ways to tune down the frequency are 1) by using a higher dielectric constant as insulation, 2) by loading the open end with a capacitor, and 3) by introducing inductance along the PIFA-strip, for instance, by giving it a meandering shape. When subsequently the PIFA concept is referred to some of these detuning means are assumed making the typical length well below λ/4.
The decreased size of the element will in all cases imply a smaller bandwidth and as a general trend the bandwidth-efficiency product (Δf/f)η will be proportional to the volume of the antenna element as expressed in the third power of the wavelength. This has a similarity with the classical Wheelers antenna size limitation stating that (Δf/f)η<13V/λ3, wherein Wheelers limitation applies for the whole radiating structure and V is the volume of the smallest sphere enclosing the structure.
The basic PIFA has a typical admittance structure (i.e. a parallel resonant circuit) when measured over the open end and by moving the input connection closer to the short-circuited end the input impedance can be adjusted to for example 50 ohms. The reactive part of the admittance (the suceptance) is not much dependent of the surrounding such as the size of the ground plane below the PIFA but it is much dependent of the stored energy within the resonant structure. A smaller distance between the strip and the ground plane will for instance give the suceptance a larger variation with the frequency around the resonance frequency where the suceptance is 0. If the strip is much shorter than λ/4 the bandwidth will likewise be smaller and in general terms the upper limit for the bandwidth will be proportional to the volume of the antenna element. The real part of the admittance (the conductance) is very important for the SPAs and the bandwidth will be proportional to the conductance, which is mainly radiation conductance with losses as an undesired additional component.
However, one problem with a typical PIFA is that the radiation conductance is difficult to control. Another problem with the original PIFA is that it is not well suited for multi-band applications, partly because the PIFA by definition is planar.
The last few years several PIFAs modified for dual-band telephone service has entered the market. The by far most common principle is to remove parts of the conduction surface to create a second resonance around for instance 1800 MHz beside a first one around 900 MHz. One basic type of modified PIFA is the so-called C-PIFA, wherein in a figurative way a thick “I” is replaced by a “C” formed by cutting away a part of a square or rectangular conduction surface, see FIG. 5. Typically the same relative bandwidth is obtained at the upper band as at the lower which is what is needed for example GSM 900/1800. The result is however much less than could be expected by Wheelers limitation. Thus, in the modified PIFA a large number of different patterns derived from I and C quoted above have been used but more or less of them have given limited results. The typical need for the higher frequency bands for a modern mobile phone is much bigger. For instance, not only GSM 1800 should be covered but also GSM 1900 and UMTS making 1700-2300 MHz a desired band. This is 30% relative bandwidth as compared to 8% for GSM 900 or GSM 1800.